Make balance and daily budget react to start date

The balance and the daily budget now react to budget start date changes
appropriately.

- Implement recalculate values method that is called when the user changes
  the budget starting date. The method figures out the new budget starting
  and ending dates, resets the daily budget appropriately and sets a new
  daily budget sum according to the new starting and ending dates.

- Implement getAmountOfDaysTillBudgetEnd method.